The present invention relates to an AC powered field mounted process control transmitter, such as a magnetic flowmeter or a coriolis flowmeter for sensing fluid flow. More particularly, the present invention relates to an inrush current limiter for use in such a field mounted transmitter.
A magnetic flowmeter is a segment of pipe which measures the velocity of a fluid passing through it. The flowmeter creates a magnetic field in the fluid which induces an electric field in the fluid. The size of the electric field induced in the fluid is proportional to the velocity of the fluid through the pipe. By measuring the induced electric field, the magnetic flowmeter measures the velocity of the fluid flowing through the pipe. The flowmeter creates the magnetic field by passing a current through coils encircling a pipe through which conductive fluid flows. The magnitude of the field is given by Ampere's Law and is perpendicular to the flow of fluid through the pipe. Two electrodes flush-mounted on opposite sides of the flow pipe measure the electric potential induced in the fluid. An AC powered magnetic flowmeter converts an AC voltage source to a DC voltage source in order to power internal circuitry. Typically, the magnetic flowmeter uses a rectifying bridge to rectify the voltage and a capacitor to smooth the rectified AC signal in order to create a DC power source. The filter capacitor usually has a large capacitance to remove unwanted ripple.
The size of the capacitor has created a problem when the AC power source is interrupted. For short interruptions the capacitor will remain charged and normal operation will resume as soon as the AC input is resumed. However, for long interruptions of AC power, the filtering capacitor will discharge. When AC power is resumed, the capacitor draws a large amount of current as it recharges. This inrush of current can activate circuit breakers, and have damaging effects on the rest of the magnetic flowmeter's circuits.
Methods of inrush current limiting include use of a thermistor to sense and limit current. This method inherently creates excessive heat which is not compatible with explosion proof requirements commonly used in the process control industry. Other methods typically use a SCR and dissipate excessive amounts of power even when the limiting feature is inactive.